This invention relates to methods and apparatus for conserving vapor and collecting liquid carbon dioxide for cleaning systems, more particularly to methods and apparatus for conserving vapor and collecting liquid carbon dioxide for carbon dioxide dry cleaning systems.
Organic solvents such as perchloroethylene and other low-pressure liquid solvents have long been popular for use in cleaning systems such as dry cleaning systems. Recently, however, there are growing concerns that these solvents may harm the environment and pose occupational safety hazards. These concerns have led to an extensive search for alternative solvents that are less hazardous and systems for applying such solvents.
Some of this research has focused on systems utilizing solvents that are gases at low pressure. These systems may operate either under subcritical conditions such that the solvent is present as a liquid or under supercritical conditions such that the solvent is present as a supercritical fluid. Some of these systems utilize liquid carbon dioxide (CO2) as a cleaning solvent.
PCT Publication WO 99/13148 to Shore et al. describes a cleaning system using liquid CO2. Shore describes evacuating a cleaning chamber to remove air from the chamber. Shore also discusses filling the chamber with carbon dioxide gas from either a distillation vessel or a liquid CO2 storage tank as part of a prefill mode. Shore further describes how draining liquid carbon dioxide from the cleaning chamber leaves carbon dioxide gas in the chamber and discusses an apparatus for reclaiming this gas using a compressor and a condenser to return reliquified CO2 to a liquid storage tank.
The system described by Shore is inefficient making it expensive to operate and expensive to construct. For example, filling the cleaning chamber with CO2 gas from a distillation vessel requires that a distillation vessel be supplied and operated. Alternatively, using vaporization of the liquid CO2 in the storage tank requires the storage tank to contain a heater sized to provide make-up heat equal to the heat of vaporization of the liquid CO2 that is converted to vapor.
Furthermore, a condenser must be supplied which is sized to handle the extreme vapor loads experienced at the beginning of the vapor reclamation step. Additionally, cooling must be supplied to this condenser. Other methods for removing the CO2 gas from the cleaning chamber such as venting to atmosphere, which results in loss of CO2 from the system, or sparging as described in PCT Publication WO 97/33031 to Taricco are similarly inefficient.
A small amount of air in the system may be beneficial, providing a partial pressure in the liquid CO2 storage tank and resulting in increased net positive suction head for the pump. However, the efficiency of the condenser can be drastically affected by even small amounts of air. Thus, a vacuum pump must be operated before each cycle to ensure that all air has been evacuated from the cleaning chamber.
Further inefficiencies occur in carbon dioxide cleaning systems that employ cleaning solutions comprising liquid carbon dioxide and other additives or detergents. To create a source of liquid CO2, these systems rely on evaporators or stills to separate additives and contaminants from the cleaning solution and generate CO2 vapor. Such stills and evaporators require heating elements, which must be sized to supply sufficient CO2 vapor and operated using steam or electricity.
It is therefore an object of the present invention to provide methods and apparatus for improving the thermodynamic efficiency of a liquid carbon dioxide dry cleaning system.
It is another object of the present invention to provide methods and apparatus for lowering the capital costs associated with a liquid carbon dioxide dry cleaning system.
These and other objects are provided, according to the present invention, by an apparatus for conserving carbon dioxide vapor in a carbon dioxide dry cleaning system employing a liquid carbon dioxide cleaning solution to clean articles, where the apparatus includes a wash tank for contacting the articles to be cleaned with the liquid carbon dioxide cleaning solution, a working tank for storing liquid carbon dioxide cleaning solution, a vapor tank for storing carbon dioxide vapor, a first piping system providing fluid communication between the wash tank and the vapor tank where the first piping system includes a first line and a first valve residing in the first line, and a second piping system providing fluid communication between the working tank and the wash tank.
According to the present invention, the first valve may be sized to limit vapor flow rate through the first line.
In a preferred embodiment, the apparatus includes a compressor for transferring carbon dioxide vapor between the wash tank and the vapor tank, where the compressor resides in the first piping system, a third piping system providing fluid communication between the working tank and the first piping system, and a condenser for condensing carbon dioxide vapor to liquid carbon dioxide, where the condenser resides in the third piping system.
According to the present invention, a method for conserving carbon dioxide vapor in a carbon dioxide dry cleaning system employing a liquid carbon dioxide cleaning solution to clean articles may also be employed, which includes removing carbon dioxide vapor from a wash tank to a vapor tank, storing the carbon dioxide vapor in the vapor tank and charging the wash tank with carbon dioxide vapor from the vapor tank. By conserving the carbon dioxide vapor, a condenser may not be needed, which may reduce or eliminate the need to remove air from the system at the beginning of each wash cycle. Thus, the need for a vacuum pump may be reduced or even eliminated resulting in lower capital costs and operating expenses. Furthermore, higher concentrations of air in the system may increase the efficiency of the system by providing a partial pressure in the head-space of the working tank, resulting in increased net positive suction head for a pump.
In a preferred embodiment, removing carbon dioxide vapor from a wash tank to a vapor tank includes transferring carbon dioxide vapor from the wash tank having a higher pressure to the vapor tank having a lower pressure utilizing a piping system, pumping the carbon dioxide vapor out of the wash tank using a compressor when the differential pressure between the wash tank and the vapor tank is less than about 100 psig, condensing a portion of the carbon dioxide vapor into liquid carbon dioxide in a condenser, storing the liquid carbon dioxide in a working tank, stopping the compressor when the pressure in the wash tank is less than about 100 psig, and venting carbon dioxide from the wash tank to atmosphere. Charging the wash tank with carbon dioxide vapor from the vapor tank includes transferring carbon dioxide vapor from the vapor tank having a higher pressure to the wash tank having a lower pressure utilizing a piping system, pumping the carbon dioxide vapor out of the vapor tank using a compressor when the differential pressure between the vapor tank and the wash tank is less than about 100 psig, generating carbon dioxide vapor in a working tank, stopping the compressor when the pressure in the wash tank is less than about 50 psig, and venting carbon dioxide from the wash tank to atmosphere.
By condensing only a portion of the carbon dioxide vapor, the size of the condenser may be reduced resulting in lower capital costs and the heat removed from the condenser may be reduced resulting in increased thermodynamic efficiency.
According to the present invention, an apparatus may also be employed for collecting liquid carbon dioxide in a carbon dioxide dry cleaning system employing a liquid carbon dioxide cleaning solution to clean articles, where the apparatus includes a vapor tank, a condenser, a working tank containing carbon dioxide cleaning solution, a wash tank, a liquid carbon dioxide collecting tank, a first piping system providing fluid communication between the condenser, the working tank, and the liquid carbon dioxide collecting tank, a second piping system providing fluid communication between the liquid carbon dioxide collecting tank and the wash tank, and a third piping system providing fluid communication between the wash tank and the vapor tank.
According to the present invention, a method may also be employed for supplying a liquid carbon dioxide solution to a wash tank for a carbon dioxide dry cleaning system, utilizing a vapor tank, a condenser, a liquid carbon dioxide collecting tank, a working tank containing carbon dioxide cleaning solution, and a wash tank, where the method includes draining a solution comprising liquid carbon dioxide from the wash tank leaving carbon dioxide vapor in the wash tank, transferring the carbon dioxide vapor from the wash tank to a vapor tank, condensing a portion of the carbon dioxide vapor transferred to the vapor tank to form liquid carbon dioxide, collecting the liquid carbon dioxide in the liquid carbon dioxide collecting tank, and draining the contents of the liquid carbon dioxide collecting tank into the wash tank. By conserving the carbon dioxide vapor left in the wash tank after draining a solution comprising liquid carbon dioxide, transferring this vapor from a wash tank to a vapor tank, and condensing a portion of this conserved carbon dioxide vapor to form liquid carbon dioxide rather than generating carbon dioxide vapor in an evaporator or the like, the need for an evaporator and like equipment may be reduced or eliminated, which may reduce capital and operating costs and may improve the thermodynamic efficiency of the cleaning system.
In a preferred embodiment, the method includes rinsing articles in the wash tank with liquid carbon dioxide after the draining step and emptying the contents of the wash tank into the working tank.
In yet another preferred embodiment, the method includes injecting additives into the liquid carbon dioxide collecting tank to form a filter wash solution after the collecting step and before the draining step, washing at least one filter with the contents of the liquid carbon dioxide collecting tank after the draining step, and emptying the wash tank.
Methods and apparatus according to the present invention may therefore improve the thermodynamic efficiency of and reduce the capital costs associated with a liquid carbon dioxide dry cleaning system. It will be understood that the present invention may be embodied as methods and apparatus and combinations thereof.